Designers of power output devices, e.g., automobile, aircraft and locomotive engines/drive trains, are faced with a myriad of competing design criteria that result in various design compromises. For example, to optimize power output, a compromise in engine torque is often necessary. Similarly, when optimal energy conversion is desired, e.g., specific fuel consumption, a designer must choose an appropriate air-standard combustion cycle to match the requirement. In yet other examples, if it is desirable to optimize horsepower and torque, one may need to accept weight and/or fuel consumption penalties.
While there are literally thousands of internal combustion engine designs and variations thereof, all employ certain fundamental or basic principles. These include: (i) the intake of a combustible mixture, e.g., oxygen and gasoline, into a working volume (ii) compression of the combustible mixture, (iii) ignition of the mixture to effect its expansion, (iv) capturing the energy in the expansion to produce work, and (v) expelling combustion by-products into an exhaust system so as to prepare the working volume for a subsequent combustion cycle.
Internal Combustion Engines (ICEs)
Commercially viable internal combustion engines include reciprocating-piston, rotary, and gas-turbine type engines. The following is a brief discussion of each followed by the advantages and disadvantages of each. Applications of each are also discussed.
Traditional reciprocating internal combustion engines employ the reciprocating motion of a piston/cylinder to perform the functions described above. Linear motion of the piston is translated into rotational motion by means of a piston rod that is articulately mounted to the underside of the piston at one end thereof and pivotally mounted at the other end to an eccentric portion of a crankshaft. Generally, these internal combustion engines employ the Clerk two-stroke, Otto four-stroke or Diesel two and/or four-stroke air-standard cycles.
A Clerk two-stroke engine typically employs a piston/cylinder arrangement wherein the cylinder comprises inlet and exhaust ports located on opposite sides of the cylinder walls. The inlet port is located slightly above the lowest point of piston travel, e.g., bottom dead center, and the exhaust port is located on the opposing side about midway relative to the piston travel. The combustible mixture is first introduced into the cylinder chamber as the piston uncovers the inlet port. The downward motion of the piston from the prior stroke, causes the combustible mixture (located within the housing) to be pressurized thus being forced into the cylinder chamber. As the piston moves upwardly and past the exhaust port, the combustible mixture is compressed and ignited upon reaching the top of the piston's stroke, i.e., top dead center. The expansion of the combustible mixture produces a downward power stroke and, upon passing the exhaust port, begins to be expelled from the cylinder chamber. As the piston travels downward yet further, the combustible mixture is pressurized by the chamber along the underside of the piston (traveling downwardly). This small quantity of pressurized air-fuel is injected into the cylinder as the piston passes the inlet port. The injection of this pressurized air-fuel mixture augments the expulsion of exhaust gases through the exhaust port and a new cycle begins. The Clerk two-stroke produces a power stroke for every two-strokes of the piston, or once per revolution, i.e., of the drive shaft. For equivalent displacement, the Otto cycle produces more power per stroke than the Clerk, but the Clerk produces more power per revolution because it has twice the number of power strokes per rev. However, the Clerk is not twice as powerful as an Otto engine of the same displacement because of its lower efficiency per stroke.
The Otto four-stroke engine is more efficient than the Clerk two-stroke engine and employs a piston/cylinder arrangement wherein the inlet and exhaust ports are located at the top/uppermost portion of the cylinder. The ports include conical, plug-shaped valves that are operated, i.e., opened and closed, by means of a drive cam. The combustible mixture is introduced into the combustion chamber by opening the inlet valve whereby the downward motion of the piston generates a vacuum for drawing the combustible mixture into the cylinder/chamber through the inlet port. In the next stroke, or the compression stroke, the inlet and exhaust valves are closed and the combustible mixture is compressed as the piston traverses upwardly. At or near the top of the piston stroke, Top Dead Center (TDC), the air-gas mixture is ignited to expand the gaseous mixture and drive the piston downwardly in the chamber. Upon completion of the downward or power stroke, the exhaust valve is opened such that the subsequent upward stroke of the piston expels the combusted gases outwardly through the exhaust port into an exhaust manifold. The Otto four-stroke cycle produces a power stroke for every four-strokes of the piston, or once every two revolutions of the drive shaft.
More recent innovations include the Wankel Rotary engine which employ the eccentric motion of a polygonal-shaped rotor within a substantially elliptically-shaped, or more accurately, a epitrochoidally-shaped housing or chamber. The point or apex of the polygonal shaped rotor creates discrete chambers that, (i) accept an air-gas mixture, (ii) expand the air-gas mixture to drive the rotor about an axis and (iii) expel the exhaust gases as the rotor passes an exhaust port. More specifically, as the rotor passes the inlet port, the combustible mixture is introduced within one of the chambers. As this chamber rotates approximately 90 degrees, the mixture is compressed against a wall of the epitrochoidally-shaped housing. Ignition plugs are located at this angular position and, upon ignition, the combustible mixture expands causing the rotor to be driven approximately 120 degrees within the housing. An exhaust port is located at the next rotational position and the combusted gases are expelled from the chamber. Similar to the compression stage of the Wankel cycle, the chamber is reduced in volume, i.e., against the wall of the epitrochoidally-shaped housing to expel the exhaust gases outwardly.
In conventional turbine engines, a compressor section is used to compress air into a combustion chamber. Fuel is introduced into the chamber and ignited to expand the air-gas mixture. Turbine vanes capture the energy of the expanded air-gases to drive a turbine shaft which also drives the compressor section to continue the combustion cycle. Generally, this form of internal combustion engine employs the Brayton air-standard cycle. From its brief description, it will be appreciated that the turbine engine is, perhaps, the most elegant, however, it too has disadvantages which limit its application.
Advantages/Disadvantages & Practical Applications of Reciprocating Piston ICEs
The following is a brief examination of reciprocating piston ICEs in terms of their properties, performance, and practical applications. Inasmuch as the Wankel rotary and turbine engines are not widely employed or have specific/limited applications, these will only be mentioned in terms of their need for gear reduction apparatus to lower output velocities to useable speeds. Furthermore, these engine designs represent a significant departure from the elements and teachings of this invention.
A two-stroke, Clerk cycle, engine delivers a power stroke with each revolution as compared to the four-stroke cycle which delivers a power stroke with every two revolutions. Consequently, reciprocating ICEs employing a two-stroke cycle can, in theory, deliver twice the power of a four-stroke. This theoretical ratio is not realized in practice however because of the lower efficiency of the power stroke in a two-cycle engine. Two-stroke engines do not require valves and the associated mechanisms for the intake of fuel and exhaust of combusted gases. Four-stroke engines, on the other hand, require a complex array of cam-driven valves for intake and exhaust. ICEs employing a two-stroke cycle can operate at any orientation, which can be important in applications wherein the powered-vehicle or device pitches or rolls such as an acrobatic fixed-wing aircraft, helicopter or chainsaw. Engines employing a four-stroke cycle require that oil be delivered from a gravity-based sump. Consequently, four-stroke engines typically are designed with the forces of gravity in mind. Two-stroke engines, therefore, offer simplicity and a significant power-to-weight ratio as compared to many four-stroke engine designs.
Disadvantages of the two-stroke air-standard cycle generally involve wear, fuel efficiency and pollution. The lack of a dedicated lubrication system typically results in a high rate of component wear. Further, two-stroke reciprocating engines, which employ a conventional crankshaft, also experience accelerated wear of the piston. To better understand this phenomenon, it should be appreciated that the eccentricity of the crankshaft causes the piston rod to be oriented off-axis relative to the piston/cylinder axis. As such, a lateral component of the resultant force vector imposes high frictional forces between the piston and cylinder. Consequently, the piston rings wear, pressure is reduced i.e., causing blow-by, and fuel efficiency decreased. Other disadvantages are simply due to the way fuel is burned (or not burned) in two-stroke engines. For example, the exhaust phase of the cycle is, at least in part, combined with the fuel intake and compression phase of the cycle. Consequently, exhaust gases are intermixed with a fresh charge of air-gas, hence, the mixture for ignition is non-optimum, i.e., contaminated by exhaust gases from the previous stroke. Similarly, inasmuch as the intake and exhaust occur nearly simultaneously, but along opposite sides of the cylinder, fresh fuel may be exhausted before ever being compressed and ignited. Consequently, two-stroke engines are not highly fuel efficient.
The principle advantage to a four-stroke air-standard cycle (or Otto cycle) relates to fuel efficiency. More specifically, four-stroke engines employ a stroke entirely dedicated to the exhaust of combusted gases, hence, four-stroke engines burn cleaner and more efficiently than two-stroke engines. That is, combusted gases do not intermix with a fresh charge of air-fuel in or during the compression/ignition stroke. Furthermore, four-stroke engines can have independent/dedicated oil and fuel reservoirs, i.e., do not use a gas-oil mixture, hence four-stroke engines experience less wear and are less costly to operate.
The disadvantages of four-stroke engines have been discussed above, i.e., when being compared to a two-stroke engine, however, suffice it to say that four-stroke engines deliver significantly less power output than two-stroke engines of the same displacement.
Diesel two and four-stroke cycles have the same advantages and disadvantages as those discussed above in connection with the Clerk and Otto air-standard cycles. Diesel engines do, however, allow high compression ratios inasmuch as the flash point (i.e., the temperature at which the fuel ignites) of Diesel fuel is substantially higher than conventional gasoline fuel. While this can offer the advantage of high power output, the advantage of Diesel engines relates to the higher specific energy of diesel fuel and the relatively high efficiency with which it burns.
All of the above air-standard cycles and engine designs operate efficiently at relatively high rotational speeds. For example, a gas turbine engine is typically efficient at about ten-thousand (10,000) RPM. Four-stroke automobile engines are efficient within a range of about fifteen hundred to three thousand (1,500 to 3000) RPM. This is also true for the Wankel rotary engine. Typically, such rotational velocities are significantly above useful speeds to, for example, drive automobile tires, helicopter rotors, ship propellers etc. Consequently, all require speed reduction devices, e.g., transmissions, to lower and control the speed of output drive shafts. It will be appreciated that such devices add significant weight, require periodic maintenance, and are costly to fabricate, operate and maintain.
Other disadvantages of ICEs of the prior art relate to the weight distribution of conventional designs and to a lack of balanced torque output. With respect to the former, the center of gravity (C.G.) of prior art ICEs is frequently offset with respect to the output shaft axis. While this does not present difficulties in many applications, in other applications, such as a compound helicopter, it is beneficial to have the engine C.G. coincident with the output drive shaft axis. For example, helicopters typically are designed such that the turbine engines are juxtaposed relative to the helicopter transmission. Despite the output orientation of the turbine engine (which faces forward), a rather elaborate bevel gearing system is employed to ensure that the center of gravity of the turbine engine lies in the same plane (normal to the longitudinal axis of the helicopter). As such, this drive-train configuration is non-optimal in terms of weight and is highly mechanically complex.
With respect to the latter, helicopters typically employ anti-torque devices to counter-act the torque developed in the fuselage as a result of the high torque required to drive the main rotor system. Conventional anti-torque devices employ tail rotors or propulsive thrusters to generate a torque vector, i.e., at a calculated distance from the main rotor driveshaft axis, which is equal and opposite to the engine-generated torque vector. As such these devices, which include tail cone structures, tail drive shafts, tail rotor gearboxes also add unnecessary weight.
The drive trains used in combination with such ICE's typically employ a speed reduction transmission that may employ bevel, helical and conventional spur gears. Such transmissions may employ a planetary gear system having a sun gear, a stationary ring gear surrounding the sun gear and a plurality of planetary pinions disposed between the sun and ring gears. A carrier assembly maintains the relative position of the planetary pinions as the sun gear drives the planetary pinions about the ring gear. In this arrangement, the sun gear is the input while the carrier assembly functions as the output. Alternatively, the carrier assembly may operate as the input to drive the sun gear.
Another prior art drive train system employs a pair of lobed cams driven by plurality of reciprocating pistons. The cams may be driven in the same direction or in opposite directions. Furthermore, the output drive shafts may be coaxial and counter-rotating to counterbalance the torque produced by each of the drive shafts. Ideal applications for such drive trains include rotorcraft such as helicopters, gyrocopters, and unmanned aerial vehicles (UAVs), submersible vehicles, and ducted fan/vectored-thrust, propeller driven boats such as those employed in the Florida Everglades.
An example of such a cam driven transmission is described and illustrated in O'Neill U.S. Pat. No. 7,219,631 entitled “High Torque, Low Velocity, Internal Combustion Engine” which is incorporated herein by reference in its entirety. The cam driven transmission described therein employs a pair of cam plates each having a lobed cam profile driven by a plurality of reciprocating pistons. The pistons are driven radially by a two or four-stroke piston-cylinder and are timed to engage a sloping drive surface of the cam profile with each downward power stroke of the piston. While the lobed cams may be driven in the same rotational direction, one particularly useful operating mode involves counter-rotation of the cams to equilibrate the torque imparted to each of the cams. As such, the torque imparted to each of the counter-rotating drive shafts is balanced resulting in cancellation of the reaction torque normally carried by the engine case.
The ability of the lobed-cam to drive torque in opposite directions facilitates a number of useful applications, however, one difficulty that can arise relates to the inability to ensure counter-rotation of the cam plates. That is, should the start-up or initial rotational position of the cams be such that the pistons are aligned with the top or bottom-dead center position within the cam profile, it may be impossible to initiate rotation of each in opposite directions without some external force or rotary drive input. In the prior art, a timing gear, e.g., a bevel gear, was employed at the periphery of each cam to establish an optimal or, at least favorable, initial position of the piston drive shafts relative to the cam surfaces, i.e., along the power stroke. As such, this requirement prohibits certain operating conditions and, in certain instances, prohibits or inhibits certain applications.
A need, therefore, exists for a lobed-cam transmission which eliminates the requirement for torque drive augmentation of one or both cam plates to ensure counter-rotational torque drive irrespective the initial rotational position of the piston drive shaft relative to the power surface of the respective cam. Other objectives include a power drive train that maximizes energy conversion, eliminates the need for torque augmentation devices, improves performance, enhances reliability, reduces weight and minimizes mechanical complexity.